One solution is to set the containers up on large landscape blocks so they don't draw moisture from the soil. I have to do that with my clay pots. Learned that the hard way by having my beautiful yellow crown of thorns get root rot and die.

I don't think it's necessary to measure more than the first few inches of soil. If that is dry, then it needs watering. I literally just stick my finger in down to the the second knuckle, and if it's dry that far down, I water.

My container veggies are not drying out as quickly as I'd like these days. I actually put holes around the sides of my 5-gallon buckets about an inch up from the bottom to keep water from pooling in the bottom. They have holes in the bottom, but just for extra drainage.

I'm not an expert on this, so maybe someone will come along who has used these containers and help you more.

DG: Thanks for your comments. Here's what my better half built for me:

Those Lowes' paint buckets were what I used the first year I grew tomatoes, and I became weary of having to water them twice a day in the heat, so I switched to plastic utility baskets which were supposedly 11 gallons, but they, too, seemed inadequate and in the end quite flimsy.

What say others about being concerned only with dryness within the first few inches of soil?

I don't think you should have a problem. You have adequate air circulation all around the baskets. They shouldn't stay too wet.